Typically, preforms for quartz glass tubing, rods, or collapsed offline rods-in-cylinders (RICs) are produced by introducing a quartz glass component (e.g., a cylinder, an ingot, or an uncollapsed RIC) into an apparatus including a heating zone (e.g., a furnace) in a vertical orientation such that the lower end begins to soften and form a strand. The strand is then placed in a pulling device including one or more sets of pulling wheels. The rate of draw of the strand is controlled by the speed of the pulling wheels, which may apply either a downward or upward force depending on the forming zone temperature or viscosity and the weight of the strand supported by the wheels. Forming is accomplished without the aid of a die thus the strand dimensions are controlled by the feed rate of the quartz glass component, the temperature of the heating zone, and the speed of the pulling wheels.
Alternatively, the preform may be formed by fusing quartz sand by electricity or by flame in a furnace and extracting a strand from the furnace by pulling through a die. The strand is then placed into the pulling device as described above.
In either case, due to the small contact areas between the pulling wheels and the preform, the amount of force that can be applied to the preform by the pulling wheels is limited because excessive pressure can damage the glass surface of the preform. For large preforms that require a greater pulling force than can be applied by a single set of pulling wheels, multiple sets of pulling wheels may be applied to the strand at different levels to achieve the necessary total force. However, multiple sets of pulling wheels increase both apparatus height and cost. Further, low bow can only be achieved with multiple sets of pulling wheels if the sets of pulling wheels are precisely aligned, which is difficult to achieve in practice.
Referring to FIG. 1A, bow is understood to be a degree of curvature in the glass component, such as glass rod 10. In the example depicted in FIG. 1A, the bow for the glass rod 10 with a single radius of curvature can be defined in terms of a horizontal offset x per length Z, or more commonly, by the maximum “camber” (deviation from the straight line “chord”) of x/2 over the length Z, all typically expressed as millimeters/meter (mm/m). However, bow may also be described as a radius R of a curvature 12 for any point along the glass component. A greater radius of curvature reflects a lesser degree of bow present for that point of the glass component. In fact, it can be easily shown that to a very good approximation R=Z2/(2*x) (Equation 1).
Referring to FIG. 1B, a glass component is formed by pulling a molten strand 15 as described above at a linear downward velocity V. This results in a frozen strand length V*t after a time t. However, if the molten strand 15 experiences a transverse or perpendicular force to the glass draw direction V with an acceleration V⊥′ (i.e. the time derivative of the transverse velocity V⊥) then bow can form and be frozen into the strand. As depicted in FIG. 1C, if V⊥ is zero (which also means V⊥′ is zero), then after a time t the strand has a length of V*t but the offset x (=V⊥*t) remains at zero so the strand is straight and does not have any bow. However, if V⊥′ is not zero, then after a time t the strand has a length of V*t but the bottom of the strand is offset by a distance ½ V⊥′*t2, as depicted in FIG. 1D. In the case where both the draw speed V and the transverse acceleration V⊥′ are constant, the radius R of the bow in strand 15 will remain constant and it is equal to V2/V⊥′. This can be derived from Equation 1 as follows:Z=V*t x=½ V⊥′ t2 R=Z2/(2*x)=V2/V⊥′  (Equation 2)
Accordingly, bow is minimized by minimizing the amount of the perpendicular or transverse acceleration V⊥′ or equivalent perpendicular or transverse force experience by the molten strand before it is frozen. It is noted that while the pulling direction V is typically substantially vertical to minimize any gravitational contributions to V⊥′, the pulling direction V may be in directions other than strictly vertical as long as the transverse force or acceleration is minimized for low bow. In fact, if V⊥′ is zero but both V and V⊥ are constant the strand can still be pulled straight without bow or curvature in a non-vertical direction with non-zero offset x (=V⊥*t).
In general, the bow or curvature of the glass component in any drawing process is simply a frozen record of how the glass is drawn or flows out of the furnace. No matter how complicated the preform or tube curvature is, at any given point (with an infinitesimal length) on the curvature it can always be described by a vector whose magnitude is the radius of curvature and whose direction is defined by the normal vector of the plane that contains the curvature of the infinitesimal length. This is a mathematically rigorous and complete description of any curvature in three dimensional space. The radius of curvature R at any given point can be again given by Equation 2, that is R=V2/V⊥′, just like the example considered above, where V is the instantaneous glass flow velocity at the given point and V⊥′ is the acceleration or time derivative of the perpendicular or transverse velocity component V⊥ relative to the velocity vector V. These general relationships among the radius of curvature, the draw velocity and the transverse acceleration or force in Equation 2 can easily be seen by noting the close analogy of a particle of mass m moving in a circle of radius R and at speed V: the centripetal force it experiences is (m*V2/R) or the centripetal (i.e. transverse or perpendicular) acceleration is V⊥′=V2/R.
Bow is undesirable for most applications where the input component is a glass rod or preform. Many applications require welding or joining a glass rod with another glass component in a glass working lathe. Bow “runout” or “wobble” in the free end of a chucked quartz rod makes it difficult to achieve a concentric and straight weld. In the case of fiber draw, a handle is welded to the top of the preform. The preform is held by the welded handle above a draw furnace in a chuck or holder and then the bottom of the preform is lowered to the furnace melt zone. Fiber is drawn from the bottom end of the preform. As preform glass is consumed during fiber drawing, the preform is lowered continuously into the melt zone. If the preform has bow, the bottom end of the preform will follow the bow and the melt zone will be offset relative to the center of the draw furnace resulting in undesirable asymmetric glass flow in fiber draw. Also, in fiber drawing it is often desirable to have the smallest possible annular gap between the preform and a sealing mechanism on the top of the draw furnace. The clearance required is a direct function of the preform bow.
Bow is also undesirable for overclad tubes used in the manufacture of Rod-in-Tube (RIT) optical fiber preforms because bowed overclad tubes make insertion of core rod difficult. The overclad tube bow also in effect results in larger overclad gaps which tend to increase the fiber core eccentricity that is detrimental to optical fiber performance.
Recent attempts have been made to replace fixed-position pulling wheels with guide elements that attach to the strand and are able to move vertically along with the strand. An example of such an attempt may be found in U.S. Pat. No. 6,938,442. However, such devices still require precise alignment of the guide elements to ensure that bow is not introduced into the strand and are not capable of adjusting the position of the guide elements to prevent misalignment. Accordingly, an apparatus for forming glass preforms including adjustable guide elements is desirable.